1. Description of the Related Art
Numerous zig-zag sewing machines are known in which the stitching action of the needle is obtained by a swinging movement of the needle bar.
In some of these machine, the needle bar is slidingly mounted in a cradle pivoted to the frame of the machine at an intermediate point on its length and the upper end of the needle bar is subjected to the action of a control device which imposes on it a reciprocating displacement of an amplitude characteristic of that desired for the stitching movement of the needle. Swiss Patent No. 458,899 and the Japanese Kokai 51-103,547 and 51-103,548 describe precisely structures of this type.
In other sewing machines, such as those disclosed by U.S. Pat. Nos. 4,215,638 and 4,458,611, the needle bar is similarly associated with a cradle subjected to a swinging movement, but this is fixed to the frame of the machine by two universal joints, the first of which is near the upper end of the cradle and the second, which is situated at the level of the lower part of the sewing head, is generally mounted on a lever support, itself pivoted on the frame of the machine.
In a third type of sewing machine shown in particular by the Patents FR 881,686, U.S. Pat. Nos. 2,862,468, 2,932,268, 2,989,016 and 4,213,409, the sewing machine no longer comprises, strictly speaking, a cradle, the needle bar being slidingly mounted in two bearings associated respectively with a first fixed support, for the one, and with a second support, displaceable transversely to the longitudinal axis of the needle bar, for the other, these bearings being fixed to the respective support by connecting members which assure them individually, displaceability with respect to their support in at least two orthogonal directions.
In the machine of the French document cited hereinabove, the connecting members in question are formed by sorts of universal joints fixed between the bearing and a portion of the casing of the machine, for the upper bearing, and, for the lower bearing, between this bearing and the end of a translational control rod extending within the upper arm of the machine as far as a drive mechanism situated in the vertical housing of the frame.
In the machines described in the last four U.S. Patents cited, the external surface of the bearings has a spherical profile by which they are pivotingly engaged in corresponding seats of complementary shape, associated with a lever support pivoted on the frame of the machine, for the upper bearing, and integral with the frame, for the lower bearing. The stitching movement of the needle bar is obtained by alternate tilting of the upper lever.
Whatever the structure of the swinging needle bar sewing machines adverted to hereinabove, this structure is heavy and complex; it is a question, in effect, of mechanically controlled machines in which the stitching movement of the needle bar is obtained by the operation of linkage systems and mechanical mechanisms responsible for transforming the movement, which is initially rotational, of a drive device, into axial displacements of adjustable amplitude according to the amplitude desired for the stitching movement of the needle bar.
Constructions of this type are however no longer acceptable in modern sewing machines and, in particular, in the electronic machines in which, it is known, an optimum compromise between the overall dimensions of the casing and a reduced weight must be achieved for an increased operational quality. On this latter point, it is to be noted that the natural inertia presented by the known mechanical adaptations representing the state of the art previously adverted to is such that it becomes practically impossible to control the movement with great speed by means of commercial stepper motors without having to oversize these latter and, because of this, also oversizing the frame which must incorporate them.
It is also to be noted that these known adaptations are of a structural complexity unacceptable in this age, as much from the point of view of the manufacture of the parts they contain as, and especially, from the point of view of, on the one hand, their assembly in the factory, and, on the other hand, their repair, and indeed their possible replacement by the retailers.